In recent years, electrochemical devices used as driving power sources have been further developed for use as a type of important key devices. Among such electrochemical devices, especially, nickel hydride storage batteries and lithium ion secondary batteries have been widely used as driving power sources for a wide range of devices including cellular phones, consumer devices, electric cars and electric tools, because they are light in weight and compact in size and also have high energy density. Recently, particular attention has been drawn to lithium ion secondary batteries as driving power sources, and the development of lithium ion secondary batteries has been actively conducted for the purpose of increasing capacity and output thereof.
A secondary battery used as a driving power source is required to provide a large output current. Therefore, secondary batteries having an improved battery configuration, specifically, an improved current collector configuration have been proposed.
For example, a current collector configuration in which, in order to increase an electrode area, a positive electrode plate and a negative electrode plate are wound with a separator interposed therebetween and each of the positive electrode plate and the negative electrode plate is jointed to a corresponding current collector plate via a current collector tab by welding or like technique has been employed. However, in such a current collector configuration, since an electric resistance of the current collector tab is large and only low current collector efficiency is achieved, it has been difficult to use this current collector configuration as a driving power source which is required to provide a large output current.
In a tab-less current collector configuration in which each of a positive electrode plate and a negative electrode plate is, in its entire area, jointed to a corresponding current collector plate, an electric resistance can be reduced, and therefore, the tab-less current collector is suitable for large current discharge. However, to achieve tab-less current collection, respective end portions of the positive electrode plate and the negative electrode plate have to be jointed to current collector plates, respectively, by welding or the like.
FIG. 7 is a plan view illustrating a tab-less current collector configuration described in Patent Document 1. As shown in FIG. 7, an electrode plate end portion 101 of an electrode group 100 wound with a separator (not shown) interposed between electrodes is welded to a current collector plate 103 having a cross-shape (a Y-shape, an I-shape or the like) at joint spots 102.
In the current collector configuration formed in the above-described manner, the electrode plate end portion 101 can be reliably welded to the current collector plate 103 at the joint spots 102. Moreover, because the current collector plate 103 has a cross-shape, joint positions can be easily checked and also a joint state can be easily examined, so that a highly reliable current collector configuration can be achieved. Furthermore, the current collector plate 103 does not have any other extra portion but the joint spots 102, and thus weight reduction of a battery can be realized. Furthermore, almost entire part of the electrode plate end portion 101 of the electrode group 100 is not covered by the current collector plate 103 and exposed, and thus the electrode group 100 can be quickly filled up with an electrolyte when injecting the electrolyte thereinto.
Patent Document 1: Japanese Published Patent Application No. 2004-247192